1. Field of the Invention
This invention in general relates to the field of leads for the electrical stimulation of living tissue, and more particularly concerns a lead having improved means for anchoring the lead which, at the same time, permits the lead to be removed from the body with relatively minor trauma to body tissues.
2. Description of the Prior Art
Electrical stimulation of the body is an increasingly important medical procedure. For example, electrical stimulation of the heart with devices commonly known as heart pacemakers has become a routine method of treating heart block and other heart ailments, while electrical stimulation of the spinal cord has proven to be effective in relieving chronic pain. The present invention involves a lead which is particularly well-suited for use in the latter context. In this context, a lead is introduced into the epidural space which surrounds the spinal cord. One end of the lead, generally called the proximal end of the lead, is attached to a source of electrical pulses, while the other end of the lead which is within the epidural space and is generally called the distal end of the lead, includes an exposed electrode by which the electrical stimulation is applied to the spinal cord.
A problem encountered in many stimulation contexts, and in particular in the context of the stimulation of the spinal cord within the epidural space, is the dislodgment of the electrode after insertion. Many different means for anchoring the distal end of the lead, including the electrode, in or about the tissue to be stimulated, have been devised. One method that is well known for anchoring the distal end of the lead is the use in at least a portion of the distal tip of the electrode of materials which enhance fibrotic growth. These materials serve to anchor the lead by stimulating the growth of fibrotic tissue about the lead, causing the lead to become enmeshed in the fibrotic material thereby anchoring it. For example, U.S. Pat. No. 3,737,579 discloses a woven netting made of Dacron.RTM., a material which enhances fibrotic growth, which netting is affixed to the lead at a location just proximal to the electrode with the intention that fibrotic tissue will grow into the netting, thereby firmly affixing the electrode to the tissue to be stimulated, such as the heart muscle. Another known lead designed for use in the epidural space includes an area along the lead body which is roughened with a fibrotic growth-inducing material such as Dacron.RTM.. The roughening consists of tiny stiff nubs of Dacron.RTM. covering a cylindrical surface area about the outer casing of the lead.
In the case of such leads which employ fibrosis-inducing materials, the fibrotic material becomes closely interwoven with the lead-anchoring material as it grows. For example, the case of U.S. Pat. No. 3,737,579 the fibrotic growth becomes interwoven in the mesh of the netting, and in the second described type of lead, the fibrotic tissue wedges between the Dacron.RTM. nubs. Should it become necessary to remove the leads after the fibrotic growth has occurred, neither of the prior art leads can be removed without tearing away from or through the fibrotic material. This tearing process can cause trauma to the normal body tissue to which the fibrosis is attached, and also can create problems caused by the residue of fibrotic material which is torn away. This torn-away material is of particular concern in the case of leads that are implanted in the epidural space, since trauma to the spinal cord can cause considerable pain and possible permanent damage.